1. Field of the Invention
The invention relates to a heat-sensitive recording sheet with an authentic or impressed watermark and a heat-sensitive recording layer containing color formers and color acceptors, and the use thereof as a ticket.
2. Description of the Prior Art
Heat-sensitive recording sheets with authenticity-verifying security features are well known, in particular for the area of higher-value applications, for example in the case of tickets, amongst other things in the form of lottery tickets and travel tickets and, for example, also in office papers. Although a large number of authenticity-verifying security features are known, true or impressed watermarks enjoy special popularity on the market, since they also impart to the product in which they are incorporated a more valuable appearance, in addition to the authenticity verification.
Heat-sensitive recording materials with non-authentic watermarks are therefore often unsuitable to satisfy the above-described market requirements completely. Such a heat-sensitive recording material is proposed, for example, in EP-A-0 844 097, according to which the recording material has, as a first security feature, a latent image printed on its rear side and produced by means of a security ink containing a fluorescent reagent. In order to form a second security feature in the form of a waterproof image on the rear side of the heat-sensitive recording material, the security ink contains a water-repellent agent.
Heat-sensitive recording materials of the type mentioned in the introduction have been known for a long time. For example, DE-U-93 02 105 proposes a web-like carrier material for a thermosensitive recording layer with a surface of high smoothness or extremely low roughness, in which at least one authentic watermark is formed in the carrier material.
In addition, WO-A-01/09435 discloses heat-sensitive recording papers with watermarks as security features, but in exactly the same way as the text acknowledged above relates to a remote prior art inasmuch as a recording paper of two paper webs laminated together, one of which has a watermark, is neither disclosed nor suggested.
EP-A-0642 928 discloses a heat-sensitive recording material which has a detection layer on its side opposite the heat-sensitive recording layer. This detection layer has only one of the reaction components required for a color-forming reaction. A desired color reaction is initiated by applying a suitable liquid, in which the second component required for the color formation is contained. Although, in the case of papers used as a substrate, watermarks are also disclosed as an additional security feature, this text also relates to a remote prior art, since any reference to the problems when using laminated paper webs with watermarks applied thereto as a substrate for the production of heat-sensitive recording papers is lacking.
The common factor in the three proposals reproduced above is the use of substrate webs having authentic watermarks, to which a heat-sensitive recording layer is applied.
In various applications, however, this results in the disadvantage of the occurrence of what are known as “shadow images” during the thermal printing process, in which the watermarks are imaged in the thermosensitive recording layer and thus disrupt the intended text image. The reason for such shadow images is, amongst other things, the density or thickness fluctuations occurring in the carrier material because of the incorporated watermarks, which have an effect in the different sensitivities over the area of the thermosensitive recording layer. A further disadvantage of the known recording materials is their restriction to materials with a relatively low mass per unit area. Paper webs with a high mass per unit area, if they are produced in accordance with this known principle, require high drying capacities in the papermaking machine or, if these are not available, can be produced only with relatively slow-running papermaking machines. However, slow-running papermaking machines are disadvantageous on economic grounds.
Following a proposal from EP-A-0156 618, a security document to be used as a credit or check card, driver's license or else as a ticket in various forms of use comprises a substrate with a pressure-sensitive image-producing coating material which may possibly be applied only partially and which is covered by a protective layer coated on or laminated on. The substrate can contain a watermark and, in order to form a security card, can be applied to a further material.
Finally, DE-C-37 69 999 discloses a process for producing two-layer security paper with inlaid security features in the form of security threads, watermarks and the like, two layers being formed on a cylinder paper machine and the security threads being laid between them, possibly with the additional application of holes or watermarks, the paper being treated, during the sheet formation or shortly thereafter, by means of air or water jets in such a way that local fiber displacement as far as the formation of holes or watermarks is achieved. The paper layers led together are then couched and dried together. Since the document does not disclose any heat-sensitive recording layers, the problems of shadow images are not addressed.
Instead of a laminate with a plurality of substrate layers, a release paper with a large number of coatings on a base substrate is the subject of JP-A-06 328 839. The proposal made there proposes that a heat-sensitive coating A for printed image development at higher temperatures, a top coating B of high opacity, a bleaching layer C and a heat-sensitive coating D for printed image development at lower temperatures be applied in that order to the base substrate. Disclosed as a security paper, this paper permits the exclusive printed image development in the coating D during a thermal printing process at low temperatures. In this case, the printing energy is not sufficient to reach the coating A and to effect the development of a printed image there. During a thermal printing process at higher temperatures, firstly the development of a printed image in the coating A is effected. The bleaching layer C likewise activated at these higher temperatures has the effect that a printed image in the coating D is suppressed. The top layer B has the effect that the printed image of the coating A does not show through the coatings C and D.
As an application for such a release paper, it is disclosed that, for example, letters to bank customers should be produced on the coating D by using the low temperature these texts can be decoded freely. Important, secret information such as account passwords are developed only on the coating A by using the higher temperature and are not reproduced on the coating D. In order to make the information available to the customer addressed, the latter has to separate the two release layers from each other, one layer comprising the base substrate with coating A, the second layer comprising the coatings B, C and D. As a further security feature, in the context of a general reference, a watermark is disclosed, which is incorporated in the base substrate. The disadvantages of possible shadow images already discussed apply to this watermark, so that no indication as to how the object on which the present invention is based is to be achieved can be gathered from this document either.
WO93/08992 A1 has already disclosed a security ticket constructed as a laminate, which is composed of a carrier sheet consisting of paper material and a heat-sensitive recording paper which is laminated onto the carrier sheet. The heat-sensitive recording paper forms the front sheet and is to have sufficient transparency in order that a security pattern or a watermark which is applied to the carrier sheet can be detected. According to a preferred embodiment, the security feature arranged on the carrier sheet is a watermark in the form of a machine-readable bar code. An item of information applied to the carrier sheet can preferably be read by machine through the transparent or semitransparent front sheet, watermarks incorporated in the carrier sheet or security codes printed on the carrier sheet counting as equally important. Obviously, therefore, the substantial teaching of WO93/08992 A1 is to use a front sheet of a transparency which not only permits information applied to the carrier sheet to be detected through the front sheet but even machine-readable bar codes to be decoded.